1. Field of the Invention
The invention relates to an aqueous dispersion containing silicon dioxide mixed-oxide produced by flame hydrolysis and a cationic polymer, a coating pigment that contains this dispersion and an ink jet absorbing medium produced with it.
2. Discussion of the Background
The use of silicon dioxide-containing aqueous dispersions for the production of coating pigments for ink-absorbing layers in the inkjet field is known. To improve the quality, the water-resistance and the color-density of the resulting ink-absorbing layers, cationic polymers are added to these dispersions in EP-A-1013605 and DE-A-10033054. However, in highly-filled dispersions, which are particularly desired because of the improved color-matching of the resulting coating pigments, the simple addition of cationic polymers results in significant problems, for example insufficient storage stability of the dispersion.
In highly-filled dispersions, the solid particles come so close to each other that surface interaction may lead to a significant increase in viscosity and to flocculation. When cationizing the solid particles, these undesirable effects may even be reinforced. This can be explained by the fact that the quantity of cationic polymer required to cationize the particles is so large that the polymer may cause bridging of solid particles, which then leads to increased viscosity or flocculation. These highly-filled dispersions also contain significant quantities of cationic polymers, which are a further cost factor.
In addition to increasing the fill level of the dispersion, it is desirable to reduce the binder content in the coating pigment. The measures should serve to provide coating pigments with a high solids content which can be dried quickly at a given drying capacity, thus increasing the productivity of the painting machine. The ink-absorbing media obtained should also have a high color-density and a high gloss.
Ways of reducing the quantity of cationic polymers while still obtaining highly-filled, stable dispersions are therefore sought, particularly in the field of inkjet printers.